4 Answers
4

There are quite some different models on the market. Canon alone already has 5 different models I believe.

It depends what you want to photograph. If you can get very close to the subject a 50mm would be enough. If you need a bit more distance (e.g. animals/insects) the 200mm would be a better (but also more expensive) choice. There is also a special model (MP-E 65mm) that allows a variable magnification (1x-5x) but has no autofocus.

Some thoughts, a bit Canon-oriented because that is the system I am using, but this should not make much of a difference:

Focal length is the main difference between macros. Given that you want to shoot at 1:1 (otherwise you probably don't need a true macro in the first place!), focal length dictates the distance from the camera to the subject; a longer focal length gives a longer working distance.

When shooting macro, depth of field rapidly approaches zero no matter what.

The max aperture of the lens is really only of academic interest for macro use; the rules of the game change in the macro world so that a nominally f/2.8 lens ends up with a max aperture of something like f/5.6-f/8 at 1:1 magnification. Just a nice thing to know :)

Indoor or outdoor don't really matter. Exception: A weather-sealed lens will be a safer bet if it is outdoors and it is raining. Given that you have a weather sealed camera body, if not, ignore this point.

Build quality can be better in more expensive lenses. Cheaper ones are probably more than good enough, though.

A lens with a USM focus motor (or the non-Canon equivalent) are handier for general photography than those with the older "electric razor"-type focus motors or screwdrive focus. For macro you will probably use manual focus anyway so this is not particularly relevant.

Some macros have internal focusing, like the Canon 100mm f/2.8 macro. Others, like the Sigma 105mm macro, grow longer (a lot longer) as you focus closer. This can make a difference to the ease of use.

Some macros like the Canon 100mm f/2.8L or the Nikon 105mm f/2.8 VR have internal stabilization, most macros do not. This is of some slight (very limited) help in a macro photography setting but can make the lens much more useful for "normal" photography. You do have to pay for it of course.

A long macro, say 150mm and longer, may benefit from a tripod mount ring, this makes the entire setup more balanced on a tripod than if you mount the camera body to the pod. The Canon 100mm has an optional tripod mount ring but I've never felt a need for it personally.

There exist dedicated macro flash units that mount to the end of the macro lens instead of on top of the camera as normal flashes do. The obvious advantage is that the lens can be so close to the subject that it shades the on-camera flash; macro flashes do not have this problem as they are ahead of the lens. This is very far from a must-have however!

One import aspect to consider is the focal length ot the Macro lens, depending on the application. The longer the focal length, the longer the minimum working distance to the subject. If you're shooting insects, for example, you'll want a longer working distance so you don't scare the poor critters off, but you'll need better stabilisation to take out camera shake. If you take a short focal length, you shorten the minimum focal distance but you'll also be able to use the lens as a portrait lens, for instance.

Rowland is pointing out that changing the focal length doesn't necessarily mean you wanted to shoot something further away. Instead, you can use a longer focal length to increase the magnification on your subject. However, strict "macro", which is a 1:1 magnification of subject onto film/sensor, dictates that a longer focal length corresponds with a longer distance to the subject.
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Evan KrallNov 10 '10 at 15:50

You'll want a wide aperture range, f/2.8-f/32 preferably. Some people are tempted to use the macro/telephoto combos that only stop down to f/3.5 at best, and this may not be enough, if you are going for an artistically shallow depth of field. You also need to make sure the lens you buy has an acceptable magnification factor. The canon 100mm offers a 1:1 mag factor. I'm not sure the 60mm gets that high, so keep that in mind.

One other thing, you need to keep minimum focusing distance in mind. A 70-200 f/2.8 may take incredibly sharp photos and provide some amazing bokeh, its longish minimum focusing distance makes it a poor choice for macro. Example, the canon 100mm has a MFD of 1', while the 50mm's is 1.5'. You want to be able to get closer.

Most macro lenses open up to F/2.8. I don't think there's a lot of difference between F/2.8 and F/3.5 in terms of depth of field, especially since the macro lenses that don't go to F/2.8 are the ones with a longer focal length. Yes, the Canon EF-s 60 F/2.8 macro lens has a 1:1 magnification ratio.
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Dave Van den EyndeJul 15 '10 at 20:39

Yes, but if you're zoomed in, your variable f/3.5 might jump to f/5.6. At such small distances, that makes a big difference in depth of field, depending on the effect you want.
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reuscamJul 15 '10 at 20:41

When doing macro photography, you'll probably use something between f/8 and f/16. Because of the magnication (1:1), the depth of field would be incredibly small when using f/2.8 or f/3.5.
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Kristof ClaesAug 18 '10 at 9:14

-1 because with macro photography your depth of field is typically too small and you'll want to use f/8 or f/16 as @reuscam suggested.
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Alex BlackNov 10 '10 at 13:53

2

Large maximum aperture still helps in focusing.
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ImreApr 7 '11 at 14:21